Files REST endpoint: Fail with ISE on PatchListNotAvailableException

At the moment PatchListNotAvailableException results in 404 Not Found,
however this response code is only appropriate if a resource that was
specified in the URL is not found. PatchListCacheImpl throws
PatchListNotAvailableException if data is inconsistent (patch set has
no revision, computing patch list failed) or if the patch set was not
set or failed to load. These are all cases which should rather result
in a 500 Internal Server Error.

Any missing revision is already handled by the Revisions REST
collection which correctly fails with 404 Not Found in this case. The
Revisions collection parses the revision before the Files REST
endpoint is invoked, hence it is safe for the Files REST endpoint to
assume that the revision exists.

Handling PatchListNotAvailableException as 404 Not Found is bad
because it leaks internal information to the user, e.g. the response
may contain the following message:
"com.google.common.util.concurrent.UncheckedExecutionException:
org.eclipse.jgit.errors.LargeObjectException: unknown object exceeds
size limit"

Also showing an error dialog instead of the Not Found page gives
better feedback to the user about what has really happened.

Change-Id: I2ad01ece93561788120292affa03ed09d184eaa1
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListNotAvailableException.java

@@ -21,6 +21,10 @@ public class PatchListNotAvailableException extends Exception {
     super(message);
   }
 
+  public PatchListNotAvailableException(String message, Throwable cause) {
+    super(message, cause);
+  }
+
   public PatchListNotAvailableException(Throwable cause) {
     super(cause);
   }